Congestion gives clue to black holes' weight

WE KNOW they are heavy, but how exactly do you weigh a black hole? The usual way to estimate the mass of black holes orbiting stars is to look at how the star wobbles under the influence of the black hole's motion. But this method is imprecise, says Nikolai Shaposhnikov, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

Instead, Shaposhnikov and Lev Titarchuk, at George Mason University in Fairfax, Virginia, thought about the disc of material ripped from the companion star that forms around such black holes. As more matter falls in, the inner part of the disc becomes congested, "like when five lines of traffic have to merge into one", Shaposhnikov says. This clogged material oscillates, producing X-ray pulses at a rate that is related to the black hole's mass, he says.